1. Field of the Invention
The present invention relates to an apparatus and a method for driving a lamp unit, and a liquid crystal display device using the same, and more particularly, to an apparatus and a method for driving a lamp unit, and a liquid crystal display device using the same that has a safety circuit.
2. Description of the Related Art
In general, application of liquid crystal displays (hereinafter, “LCD”) has been gradually widening due to their light weights, thin size, and low power consumption. In accordance with such a trend, LCDs are used in office automation devices, audio/video devices and the like. LCDs adjust transmittance quantity of light in accordance with an image signal applied to a matrix of a plurality of control switches to thereby display desired pictures on a screen.
Since LCDs are not light-emitting display devices, they need a back light unit as a light source. There are two types of back light units for the LCD, i.e., a direct-below-type and an edge-type depending on the arrangement of a lamp. Further, there are two types of lamps for the back light unit, i.e., a cold cathode fluorescent lamp (CCFL) and an external electrode fluorescent lamp (EEFL) in accordance with the shape of the lamp.
With regard to the arrangement of the lamp, in the edge type back light unit, a lamp is installed along the exterior periphery of a flat panel and a transparent light guide plate is used to thereby transfer the light from the lamp to an entire surface of a liquid crystal display panel. In the direct-below-type back light unit, several lamps are arranged in a plane parallel to a flat panel, and a diffusion panel is installed between the lamps and the liquid crystal display panel to uniformly distribute the light from the lamps to an entire surface of a liquid crystal display panel while fixedly keeping the distance between the liquid crystal display panel and the lamps.
With regard to the shape of the lamp, in the CCFL type, power is supplied to an electrode provided on both ends of a glass tube of the lamp. In the EEFL type, power is supplied to an electrode part in which a metal material is provided on both ends of a glass tube of the lamp.
FIG. 1 is a block diagram showing a related art lamp driver 60. As illustrated in FIG. 1, the lamp driver 60 connected to a plurality of lamps 36 includes an inverter 46 to receive DC voltage Vin from an external voltage source and to convert it into an AC signal, a transformer 48 to boost the AC signal generated from the inverter 46 and to apply the boosted AC signal to lamps 36, a feedback circuit 42 to detect a current supplied from the inverter 46 to lamps 36, and a controller (e.g., pulse width modulator PWM) 44 to control the inverter 46 in accordance with a feedback signal generated from the feedback circuit 42. The transformer 48 includes a primary winding wire 51 connected to the inverter 46, a secondary winding wire 53 synchronized to the primary winding wire 51 to generate an AC signal, and an auxiliary winding wire 52 arranged between the primary winding wire 51 and the secondary winding wire 53.
The lamp driver 60 having the above structure should comply with a safety standard in consideration for the safety of a user. The safety standard requires that a current flowing through a user when lamp driver 60 is contacted should be limited to a current (mA) less than 0.7 times that of the system operating frequency. When using a single lamp unit, the single lamp is manufactured in consideration of the above safety standard. For example, if a user contact with lamp driver 60 corresponds to an unloaded 2 kΩ, an equivalent resistance element of lamp 36 corresponds to about 200 kΩ, which is a common value. If the operating frequency is 65 kHz and the lamp 36 is normally operated, then a resonance characteristic of the secondary winding wire 53 suddenly changes when 2 kΩ is contacted with the secondary winding wire 53. Generally, the secondary winding wire 53 becomes a parallel resonance.
In parallel resonance, voltage gain of an input and an output changes in proportion to a resistance element of a load. In other words, the equivalent resistance element of the lamp 36 (200 kΩ) is connected to an unloaded resistance 59 of the user (2 kΩ in parallel. Therefore, the equivalent resistance shown from the secondary winding wire 53 is about 2 kΩ (200 kΩ∥2 kΩ). Accordingly, a load change of about 1/100 is generated, so that a gain change of about 1/100 is generated. Thus, the voltage of the secondary winding wire 53 is in compliance with the safety standard.
To quantitatively verify this, the current in compliance with the safety standard of a lamp using 65 kHz frequency is 46 mA(=0.7*65). Since the gain is 1/100, the voltage of the secondary winding wire 53 is about 15V(=1500* 1/100). Therefore, the current passing through 2 kΩ becomes 7 mA in accordance with Ohm's law, thereby satisfying the safety standard (i.e., less than 46 mA).
However, in a case, for example, when ten lamps 36 are driven, for example, an equivalent resistance of the lamps 36 becomes 20 kΩ, if a user is connected to the system (i.e., if 2 kΩ, of the unloaded resistance 59 is connected to the system). The result is a gain of an output voltage becomes 1/10. Accordingly, the voltage of the secondary winding wire 53 becomes about 150V, so that the current flowing in the unloaded resistance 59 becomes about 70 mA. Thus, the safety standard is not satisfied.